SSD upgrade for iPad 1

It seems unlikely that it would work. The iPad is closer to an embedded device (like a phone) than it is to a general computing device (like a laptop or a desktop computer).

Just because the connector is close or the same doesn't mean the signaling protocols that travel over those wires are the right ones to communicate with general PCI-E devices, and for it to talk to an SSD there would need to be hard drive controller of some kind on the other end of the PCI-E bus. Since no iPad model ships with the intention of connecting to SSDs, it seems unlikely Apple would include an unnecessary chip on the logic board.

On top of all that, installing OS X on an iPad would be extraordinarily challenging. In particular, the OS is compiled for x86 and x64 processor architectures and the iPad's A4 processor is an ARM chip, so you would most likely need access the full operating system source code to recompile it for the proper architecture. Even if you got that far, there would probably be lots of hardware compatibility problems and missing drivers that would have to write and link into the OS yourself.